DPRIVE                                                          T. Reddy
Internet-Draft                                                   D. Wing
Intended status: Experimental                                   P. Patil
Expires: June 1, 2017                                              Cisco
                                                       November 28, 2016

  Specification for DNS over Datagram Transport Layer Security (DTLS)


   DNS queries and responses are visible to network elements on the path
   between the DNS client and its server.  These queries and responses
   can contain privacy-sensitive information which is valuable to

   This document proposes the use of Datagram Transport Layer Security
   (DTLS) for DNS, to protect against passive listeners and certain
   active attacks.  As latency is critical for DNS, this proposal also
   discusses mechanisms to reduce DTLS round trips and reduce DTLS
   handshake size.  The proposed mechanism runs over port 853.

Status of This Memo

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   This Internet-Draft will expire on June 1, 2017.

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   Copyright (c) 2016 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of

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   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Relationship to TCP Queries and to DNSSEC . . . . . . . .   3
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  Establishing and Managing DNS-over-DTLS Sessions  . . . . . .   4
     3.1.  Session Initiation  . . . . . . . . . . . . . . . . . . .   4
     3.2.  DTLS Handshake and Authentication . . . . . . . . . . . .   4
     3.3.  Established Sessions  . . . . . . . . . . . . . . . . . .   5
   4.  Performance Considerations  . . . . . . . . . . . . . . . . .   6
   5.  PMTU issues . . . . . . . . . . . . . . . . . . . . . . . . .   7
   6.  Anycast . . . . . . . . . . . . . . . . . . . . . . . . . . .   8
   7.  Usage . . . . . . . . . . . . . . . . . . . . . . . . . . . .   8
   8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   8
   9.  Security Considerations . . . . . . . . . . . . . . . . . . .   9
   10. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .   9
   11. References  . . . . . . . . . . . . . . . . . . . . . . . . .   9
     11.1.  Normative References . . . . . . . . . . . . . . . . . .   9
     11.2.  Informative References . . . . . . . . . . . . . . . . .  11
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  12

1.  Introduction

   The Domain Name System is specified in [RFC1034] and [RFC1035].  DNS
   queries and responses are normally exchanged unencrypted and are thus
   vulnerable to eavesdropping.  Such eavesdropping can result in an
   undesired entity learning domains that a host wishes to access, thus
   resulting in privacy leakage.  The DNS privacy problem is further
   discussed in [RFC7626].

   This document defines DNS over DTLS (DNS-over-DTLS) which provides
   confidential DNS communication between stub resolvers and recursive
   resolvers, stub resolvers and forwarders, forwarders and recursive
   resolvers.  DNS-over-DTLS puts an additional computational load on
   servers.  The largest gain for privacy is to protect the
   communication between the DNS client (the end user's machine) and its
   caching resolver.  DNS-over-DTLS might work equally between recursive
   clients and authoritative servers, but this application of the
   protocol is out of scope for the DNS PRIVate Exchange (DPRIVE)
   Working Group per its current charter.  This document does not change
   the format of DNS messages.

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   The motivations for proposing DNS-over-DTLS are that

   o  TCP suffers from network head-of-line blocking, where the loss of
      a packet causes all other TCP segments to not be delivered to the
      application until the lost packet is re-transmitted.  DNS-over-
      DTLS, because it uses UDP, does not suffer from network head-of-
      line blocking.

   o  DTLS session resumption consumes 1 round trip whereas TLS session
      resumption can start only after TCP handshake is complete.
      However, with TCP Fast Open [RFC7413], the implementation can
      achieve the same RTT efficiency as DTLS.

   Note: DNS-over-DTLS is an experimental update to DNS, and the
   experiment will be concluded when the specification is evaluated
   through implementations and interoperability testing.

1.1.  Relationship to TCP Queries and to DNSSEC

   DNS queries can be sent over UDP or TCP.  The scope of this document,
   however, is only UDP.  DNS over TCP can be protected with TLS, as
   described in [RFC7858].  DNS-over-DTLS alone cannot provide privacy
   for DNS messages in all circumstances, specifically when the DTLS
   record size is larger than the path MTU.  In such situations the DNS
   server will respond with a truncated response (see Section 5).
   Therefore DNS clients and servers that implement DNS-over-DTLS MUST
   also implement DNS-over-TLS in order to provide privacy for clients
   that desire Strict Privacy as described in

   DNS Security Extensions (DNSSEC [RFC4033]) provides object integrity
   of DNS resource records, allowing end-users (or their resolver) to
   verify legitimacy of responses.  However, DNSSEC does not provide
   privacy for DNS requests or responses.  DNS-over-DTLS works in
   conjunction with DNSSEC, but DNS-over-DTLS does not replace the need
   or value of DNSSEC.

2.  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "OPTIONAL" in this document are to be interpreted as described in
   [RFC2119] .

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3.  Establishing and Managing DNS-over-DTLS Sessions

3.1.  Session Initiation

   By default, DNS-over-DTLS MUST run over standard UDP port 853 as
   defined in Section 8, unless the DNS server has mutual agreement with
   its clients to use a port other than 853 for DNS-over-DTLS.  In order
   to use a port other than 853, both clients and servers would need a
   configuration option in their software.

   The DNS client should determine if the DNS server supports DNS-over-
   DTLS by sending a DTLS ClientHello message to port 853 on the server,
   unless it has mutual agreement with its server to use a port other
   than port 853 for DNS-over-DTLS.  Such another port MUST NOT be port
   53 but MAY be from the "first-come, first-served" port range (User
   Ports [RFC6335], range 1024- 49151) . This recommendation against use
   of port 53 for DNS-over-DTLS is to avoid complication in selecting
   use or non-use of DTLS and to reduce risk of downgrade attacks.

   A DNS server that does not support DNS-over-DTLS will not respond to
   ClientHello messages sent by the client.  If no response is received
   from that server, and the client has no better round-trip estimate,
   the client SHOULD retransmit the DTLS ClientHello according to
   Section of [RFC6347].  After 15 seconds, it SHOULD cease
   attempts to re-transmit its ClientHello.  The client MAY repeat that
   procedure to discover if DNS-over-DTLS service becomes available from
   the DNS server, but such probing SHOULD NOT be done more frequently
   than every 24 hours and MUST NOT be done more frequently than every
   15 minutes.  This mechanism requires no additional signaling between
   the client and server.

   DNS clients and servers MUST NOT use port 853 to transport cleartext
   DNS messages.  DNS clients MUST NOT send and DNS servers MUST NOT
   respond to cleartext DNS messages on any port used for DNS-over-DTLS
   (including, for example, after a failed DTLS handshake).  There are
   significant security issues in mixing protected and unprotected data,
   therefore UDP connections on a port designated by a given server for
   DNS-over-DTLS are reserved purely for encrypted communications.

3.2.  DTLS Handshake and Authentication

   DNS client initiates DTLS handshake as described in [RFC6347],
   following the best practices specified in [RFC7525].  After DTLS
   negotiation completes, if the DTLS handshake succeeds according to
   [RFC6347] the connection will be encrypted and is now protected from

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   DNS privacy requires encrypting the query (and response) from passive
   attacks.  Such encryption typically provides integrity protection as
   a side-effect, which means on-path attackers cannot simply inject
   bogus DNS responses.  However, to provide stronger protection from
   active attackers pretending to be the server, the server itself needs
   to be authenticated.  To authenticate the server providing DNS
   privacy, DNS client MUST use the authenication mechanisms discussed
   in [I-D.ietf-dprive-dtls-and-tls-profiles].  This document does not
   propose new ideas for authentication.

3.3.  Established Sessions

   In DTLS, all data is protected using the same record encoding and
   mechanisms.  When the mechanism described in this document is in
   effect, DNS messages are encrypted using the standard DTLS record
   encoding.  When a user of DTLS wishes to send a DNS message, the data
   is delivered to the DTLS implementation as an ordinary application
   data write (e.g., SSL_write()).  A single DTLS session can be used to
   send multiple DNS requests and receive multiple DNS responses.

   To mitigate the risk of unintentional server overload, DNS-over-DTLS
   clients MUST take care to minimize the number of concurrent DTLS
   sessions made to any individual server.  It is RECOMMENDED that for
   any given client/server interaction there SHOULD be no more than one
   DTLS session.  Similarly, servers MAY impose limits on the number of
   concurrent DTLS sessions being handled for any particular client IP
   address or subnet.  These limits SHOULD be much looser than the
   client guidelines above, because the server does not know, for
   example, if a client IP address belongs to a single client, is
   multiple resolvers on a single machine, or is multiple clients behind
   a device performing Network Address Translation (NAT).

   In between normal DNS traffic while the communication to the DNS
   server is quiescent, the DNS client MAY want to probe the server
   using DTLS heartbeat [RFC6520] to ensure it has maintained
   cryptographic state.  Such probes can also keep alive firewall or NAT
   bindings.  This probing reduces the frequency of needing a new
   handshake when a DNS query needs to be resolved, improving the user
   experience at the cost of bandwidth and processing time.

   A DTLS session is terminated by the receipt of an authenticated
   message that closes the connection (e.g., a DTLS fatal alert).  If
   the server has lost state, a DTLS handshake needs to be initiated
   with the server.  For the client, state should be destroyed when
   disconnecting from the network (e.g., associated IP interface is
   brought down).  For the server, to mitigate the risk of unintentional
   server overload, it is RECOMMENDED that the default DNS-over-DTLS
   server application-level idle time out be on the order of several

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   seconds, but no particular value is specified.  When no DNS queries
   have been received from the client after idle time out, the server
   MUST send a DTLS fatal alert and then destroy its DTLS state.  The
   DTLS fatal alert packet indicates the server has destroyed its state,
   signaling to the client if it wants to send a new DTLS message it
   will need to re-establish cryptographic context with the server (via
   full DTLS handshake or DTLS session resumption).  In practice, the
   idle period can vary dynamically, and servers MAY allow idle
   connections to remain open for longer periods as resources permit.

   Figure 1 shows DTLS handshake and issuing new session ticket for
   session resumption.

        Client                                          Server
        ------                                          ------

        ClientHello             -------->
        (empty SessionTicket extension)
                                       (empty SessionTicket extension)
                                <--------      ServerHelloDone

        Finished                -------->
                                <--------             Finished

        DNS Request             --------->

                                <---------  DNS Response

   Figure 1: Message Flow for Full Handshake Issuing New Session Ticket

4.  Performance Considerations

   DTLS protocol profile for DNS-over-DTLS is discussed in Section 11 of
   [I-D.ietf-dprive-dtls-and-tls-profiles].  To reduce the number of
   octets of the DTLS handshake, especially the size of the certificate
   in the ServerHello (which can be several kilobytes), DNS clients and

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   servers can use raw public keys [RFC7250] or Cached Information
   Extension [RFC7924].  Cached Information Extension avoids
   transmitting the server's certificate and certificate chain if the
   client has cached that information from a previous TLS handshake.
   TLS False Start [RFC7918] can reduce round-trips by allowing the TLS
   second flight of messages (ChangeCipherSpec) to also contain the
   (encrypted) DNS query.

   It is highly advantageous to avoid server-side DTLS state and reduce
   the number of new DTLS sessions on the server which can be done with
   TLS Session Resumption without server state [RFC5077].  This also
   eliminates a round-trip for subsequent DNS-over-DTLS queries, because
   with [RFC5077] the DTLS session does not need to be re-established.

   Since responses within a DTLS session can arrive out of order,
   clients MUST match responses to outstanding queries on the same DTLS
   connection using the DNS Message ID.  If the response contains a
   question section, the client MUST match the QNAME, QCLASS, and QTYPE
   fields.  Failure by clients to properly match responses to
   outstanding queries can have serious consequences for
   interoperability ( [RFC7766], Section 7).

5.  PMTU issues

   Compared to normal DNS, DTLS adds at least 13 octets of header, plus
   cipher and authentication overhead to every query and every response.
   This reduces the size of the DNS payload that can be carried.  DNS
   client and server MUST support the EDNS0 option defined in [RFC6891]
   so that the DNS client can indicate to the DNS server the maximum DNS
   response size it can reassemble and deliver in the DNS client's
   network stack.  If the DNS client does set the EDNS0 option defined
   in [RFC6891] then the maximum DNS response size of 512 bytes plus
   DTLS overhead will be well within the Path MTU.  If the Path MTU is
   not known, an IP MTU of 1280 bytes SHOULD be assumed.  The client
   sets its EDNS0 value as if DTLS is not being used.  The DNS server
   MUST ensure that the DNS response size does not exceed the Path MTU
   i.e. each DTLS record MUST fit within a single datagram, as required
   by [RFC6347].  The DNS server must consider the amount of record
   expansion expected by the DTLS processing when calculating the size
   of DNS response that fits within the path MTU.  Path MTU MUST be
   greater than or equal to [DNS response size + DTLS overhead of 13
   octets + padding size ([RFC7830]) + authentication overhead of the
   negotiated DTLS cipher suite + block padding (Section of
   [RFC6347]].  If the DNS server's response were to exceed that
   calculated value, the server MUST send a response that does fit
   within that value and sets the TC (truncated) bit.  Upon receiving a
   response with the TC bit set and wanting to receive the entire
   response, the client behaviour is governed by the current Usage

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   profile [I-D.ietf-dprive-dtls-and-tls-profiles].  For Strict Privacy
   the client MUST only send a new DNS request for the same resource
   record over an encrypted transport (e.g.  DNS-over-TLS [RFC7858]).
   Clients using Opportunistic Privacy SHOULD try for the best case (an
   encrypted and authenticated transport) but MAY fallback to
   intermediate cases and eventually the worst case scenario (clear
   text) in order to obtain a response.

6.  Anycast

   DNS servers are often configured with anycast addresses.  While the
   network is stable, packets transmitted from a particular source to an
   anycast address will reach the same server that has the cryptographic
   context from the DNS-over-DTLS handshake.  But when the network
   configuration or routing changes, a DNS-over-DTLS packet can be
   received by a server that does not have the necessary cryptographic
   context.  Clients using DNS-over-DTLS need to always be prepared to
   re-initiate DTLS handshake and in the worst case this could even
   happen immediately after re-initiating a new handshake.  To encourage
   the client to initiate a new DTLS handshake, DNS servers SHOULD
   generate a DTLS fatal alert message in response to receiving a DTLS
   packet for which the server does not have any cryptographic context.
   Upon receipt of an un-authenicated DTLS fatal alert, the DTLS client
   validates the fatal alert is within the replay window
   (Section of [RFC6347]).  It is difficult for the DTLS client
   to validate that the DTLS fatal alert was generated by the DTLS
   server in response to a request or was generated by an on- or off-
   path attacker.  Thus, upon receipt of an in-window DTLS fatal alert,
   the client SHOULD continue re-transmitting the DTLS packet (in the
   event the fatal alert was spoofed), and at the same time it SHOULD
   initiate DTLS session resumption.  When the DTLS client receives an
   authenticated DNS response from one of those DTLS sessions, the other
   DTLS session should be terminated.

7.  Usage

   Two Usage Profiles, Strict and Opportunistic are explained in
   [I-D.ietf-dprive-dtls-and-tls-profiles].  Using encrypted DNS
   messages with an authenticated server is most preferred, encrypted
   DNS messages with an unauthenticated server is next preferred, and
   plain text DNS messages is least preferred.

8.  IANA Considerations

   This specification uses port 853 already allocated in the IANA port
   number registry as defined in Section 6 of [RFC7858].

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9.  Security Considerations

   The interaction between a DNS client and DNS server requires Datagram
   Transport Layer Security (DTLS) with a ciphersuite offering
   confidentiality protection.  The guidance given in [RFC7525] MUST be
   followed to avoid attacks on DTLS.  DNS clients keeping track of
   servers known to support DTLS enables clients to detect downgrade
   attacks.  To interfere with DNS-over-DTLS, an on- or off-path
   attacker might send an ICMP message towards the DTLS client or DTLS
   server.  As these ICMP messages cannot be authenticated, all ICMP
   errors should be treated as soft errors [RFC1122].  If the DNS query
   was sent over DTLS then the corresponding DNS response MUST only be
   accepted if it is received over the same DTLS connection.  This
   behavior mitigates all possible attacks described in Measures for
   Making DNS More Resilient against Forged Answers [RFC5452].  Security
   considerations in [RFC6347] and
   [I-D.ietf-dprive-dtls-and-tls-profiles] are to be taken into account.

   A malicious client might attempt to perform a high number of DTLS
   handshakes with a server.  As the clients are not uniquely identified
   by the protocol and can be obfuscated with IPv4 address sharing and
   with IPv6 temporary addresses, a server needs to mitigate the impact
   of such an attack.  Such mitigation might involve rate limiting
   handshakes from a certain subnet or more advanced DoS/DDoS techniques
   beyond the scope of this paper.

10.  Acknowledgements

   Thanks to Phil Hedrick for his review comments on TCP and to Josh
   Littlefield for pointing out DNS-over-DTLS load on busy servers (most
   notably root servers).  The authors would like to thank Simon
   Josefsson, Daniel Kahn Gillmor, Bob Harold, Ilari Liusvaara, Sara
   Dickinson, Christian Huitema, Stephane Bortzmeyer, Alexander
   Mayrhofer, Allison Mankin, Jouni Korhonen and Geoff Huston for
   discussions and comments on the design of DNS-over-DTLS.  The authors
   would like to give special thanks to Sara Dickinson for her help.

11.  References

11.1.  Normative References

   [RFC1034]  Mockapetris, P., "Domain names - concepts and facilities",
              STD 13, RFC 1034, DOI 10.17487/RFC1034, November 1987,

   [RFC1035]  Mockapetris, P., "Domain names - implementation and
              specification", STD 13, RFC 1035, DOI 10.17487/RFC1035,
              November 1987, <http://www.rfc-editor.org/info/rfc1035>.

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   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,

   [RFC4033]  Arends, R., Austein, R., Larson, M., Massey, D., and S.
              Rose, "DNS Security Introduction and Requirements",
              RFC 4033, DOI 10.17487/RFC4033, March 2005,

   [RFC5077]  Salowey, J., Zhou, H., Eronen, P., and H. Tschofenig,
              "Transport Layer Security (TLS) Session Resumption without
              Server-Side State", RFC 5077, DOI 10.17487/RFC5077,
              January 2008, <http://www.rfc-editor.org/info/rfc5077>.

   [RFC5452]  Hubert, A. and R. van Mook, "Measures for Making DNS More
              Resilient against Forged Answers", RFC 5452,
              DOI 10.17487/RFC5452, January 2009,

   [RFC6347]  Rescorla, E. and N. Modadugu, "Datagram Transport Layer
              Security Version 1.2", RFC 6347, DOI 10.17487/RFC6347,
              January 2012, <http://www.rfc-editor.org/info/rfc6347>.

   [RFC6520]  Seggelmann, R., Tuexen, M., and M. Williams, "Transport
              Layer Security (TLS) and Datagram Transport Layer Security
              (DTLS) Heartbeat Extension", RFC 6520,
              DOI 10.17487/RFC6520, February 2012,

   [RFC6891]  Damas, J., Graff, M., and P. Vixie, "Extension Mechanisms
              for DNS (EDNS(0))", STD 75, RFC 6891,
              DOI 10.17487/RFC6891, April 2013,

   [RFC7525]  Sheffer, Y., Holz, R., and P. Saint-Andre,
              "Recommendations for Secure Use of Transport Layer
              Security (TLS) and Datagram Transport Layer Security
              (DTLS)", BCP 195, RFC 7525, DOI 10.17487/RFC7525, May
              2015, <http://www.rfc-editor.org/info/rfc7525>.

   [RFC7830]  Mayrhofer, A., "The EDNS(0) Padding Option", RFC 7830,
              DOI 10.17487/RFC7830, May 2016,

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11.2.  Informative References

              Dickinson, S., Gillmor, D., and T. Reddy, "Authentication
              and (D)TLS Profile for DNS-over-(D)TLS", draft-ietf-
              dprive-dtls-and-tls-profiles-07 (work in progress),
              October 2016.

   [RFC1122]  Braden, R., Ed., "Requirements for Internet Hosts -
              Communication Layers", STD 3, RFC 1122,
              DOI 10.17487/RFC1122, October 1989,

   [RFC6335]  Cotton, M., Eggert, L., Touch, J., Westerlund, M., and S.
              Cheshire, "Internet Assigned Numbers Authority (IANA)
              Procedures for the Management of the Service Name and
              Transport Protocol Port Number Registry", BCP 165,
              RFC 6335, DOI 10.17487/RFC6335, August 2011,

   [RFC7250]  Wouters, P., Ed., Tschofenig, H., Ed., Gilmore, J.,
              Weiler, S., and T. Kivinen, "Using Raw Public Keys in
              Transport Layer Security (TLS) and Datagram Transport
              Layer Security (DTLS)", RFC 7250, DOI 10.17487/RFC7250,
              June 2014, <http://www.rfc-editor.org/info/rfc7250>.

   [RFC7413]  Cheng, Y., Chu, J., Radhakrishnan, S., and A. Jain, "TCP
              Fast Open", RFC 7413, DOI 10.17487/RFC7413, December 2014,

   [RFC7626]  Bortzmeyer, S., "DNS Privacy Considerations", RFC 7626,
              DOI 10.17487/RFC7626, August 2015,

   [RFC7766]  Dickinson, J., Dickinson, S., Bellis, R., Mankin, A., and
              D. Wessels, "DNS Transport over TCP - Implementation
              Requirements", RFC 7766, DOI 10.17487/RFC7766, March 2016,

   [RFC7858]  Hu, Z., Zhu, L., Heidemann, J., Mankin, A., Wessels, D.,
              and P. Hoffman, "Specification for DNS over Transport
              Layer Security (TLS)", RFC 7858, DOI 10.17487/RFC7858, May
              2016, <http://www.rfc-editor.org/info/rfc7858>.

   [RFC7918]  Langley, A., Modadugu, N., and B. Moeller, "Transport
              Layer Security (TLS) False Start", RFC 7918,
              DOI 10.17487/RFC7918, August 2016,

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   [RFC7924]  Santesson, S. and H. Tschofenig, "Transport Layer Security
              (TLS) Cached Information Extension", RFC 7924,
              DOI 10.17487/RFC7924, July 2016,

Authors' Addresses

   Tirumaleswar Reddy
   Cisco Systems, Inc.
   Cessna Business Park, Varthur Hobli
   Sarjapur Marathalli Outer Ring Road
   Bangalore, Karnataka  560103

   Email: tireddy@cisco.com

   Dan Wing
   Cisco Systems, Inc.
   170 West Tasman Drive
   San Jose, California  95134

   Email: dwing@cisco.com

   Prashanth Patil
   Cisco Systems, Inc.

   Email: praspati@cisco.com

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